Drip edge on bottom side of window sill...

I am going through my house window by window restoring them. THey are double hung with big sills. I just noticed that the underside of the sills don't have that little groove that modern windows have, which i presume to be a drip edge. Questions are -- is it worthwhile to cut one in while I am already working on the windows, and how would I go about doing so with the sill in place? Thanks for any insights.